The invention relates to an irradiation device with a collimator used to define a beam of high-energy rays proceeding from an essentially punctiform radiation source and directed to an object to be treated, and which serves for radiation treatment, in particular the stereotactic conformation radiotherapy of tumors, whereby the collimator is realized by means of iris diaphragms with adjusting elements so as to provide for variable apertures for beam collimation, and a mechanism that shall be used to direct the beams limited by the collimator to the object to be treated from all sides, and whereby the parameters for direction, surface area, intensity and time of irradiation can be controlled in such a manner that a three-dimensional dosing profile for radiation application can be achieved.
Furthermore, the invention relates to a collimator used to define a beam of high-energy rays proceeding from an essentially punctiform radiation source and directed to an object to be treated, and which serves for radiation treatment, in particular the stereotactic conformation radiotherapy of tumors, whereby the collimator is realized by means of iris diaphragms with adjusting elements so as to provide for variable apertures for beam collimation.
Irradiation devices with collimators for the restriction of a beam of high-energy rays are used, above all, in the therapy of tumors. Collimators are also used in imaging systems, such as X-ray devices. In this context, collimators are used to restrict the beam so as to provide best possible protection of healthy tissue in the vicinity of the area to be diagnosed or treated and to avoid damage to this tissue or to reduce the damage to a minimum.
Application must be differentiated into diagnostic and therapeutic radiation. The energy of the former must be low enough to prevent damaging of the tissue, if possible. These rays are only used for the purpose of image generation, for example, in the course of irradiation treatment preparation, so that the doctor can visualize the position of a tumor, as well as the surrounding and possibly critical tissue, such as nerves. Since the surrounding tissue must be visible, some areas outside the tumor or other sections to be diagnosed must also be exposed to radiation. Since in most cases the tumors or other areas to be diagnosed are round in shape, the imaging field should be circular as well and, of course, it should not exceed the area of interest to the doctor, as X-rays are also harmful to the tissue. Therapeutic radiation, however, must have an energy level that allows the destruction of diseased tissue, such as tumors. Consequently, this radiation would also destroy the surrounding tissue. For this reason, the shielding should be suitable to limit the beam to exactly the shape of the affected tissue.
When applying high-energy therapeutic rays—which is the intended purpose of the invention-specific irradiation device and collimator—major importance is attached to the protection of the surrounding tissue, since the patient's quality of life after treatment often depends on this aspect. Side effects of irradiation through the impairment of healthy tissue cannot be absolutely excluded, however, they should be reduced to a technically feasible minimum. Since this is of essential significance and, in contrast to irradiation with X-rays, has very serious consequences even in the case of short-time exposure, the development of irradiation devices offering an increased accuracy in radiation application has been pursued by experts for many decades, so as to reduce the negative side effects of radiotherapy as far as possible.
Originally, the irradiation devices and imaging devices were equipped with collimators that could restrict the irradiation field in size but not in shape. As regards image-generating X-rays this does not imply any major consequences for the patient, since X-rays are harmful only after long exposure, which is not required for imaging, or if the patient is exposed to X-rays very frequently within a short period of time. Only high-energy therapeutic radiation—as required, for example, for the destruction of tumor tissue—will cause damage to healthy tissue in the over-range irradiation area, i.e. outside the tumor tissue to be irradiated, since radiation must be applied at a certain intensity and over a certain period of time required to destroy the tumor issue.
These over-range irradiation areas were caused by missing or insufficient imaging of the contour of the tumor tissue by the collimators and resulted from the occurrence of penumbrae at the borders of the area to be irradiated, since the shielding material was not in parallel alignment with the rays so that it could not provide full shielding, in particular when large areas were to be irradiated. With high-energy therapeutic beams, these penumbrae affected much larger areas, since the required thickness of the shielding is a multiple of that required for X-rays with a comparatively low radiation level. Furthermore, many irradiation devices showed leakage radiation emitting through the gap between adjacent shielding plates.
One example of a collimator restricting the beam only in size is an older model according to U.S. Pat. No. 2,675,486. This publication refers to a collimator for high-energy beams and equipped with four beam collimating blocks which can be displaced in one plane with the help of adjoining side faces, so that a square beam collimation in different sizes can be achieved. As tumors, however, are normally not square-shaped but have mostly a round shape, the over-range irradiation area at the corners was quite large. Moreover, large irradiation fields involved large penumbrae, since the block collimators were no longer in parallel alignment with the diverging optical path.
For this reason, experts made an effort to defuse these problems.
Based on a collimator of the type described above, the publication DE 20 53 089 A1 suggests, for the field of X-ray imaging, use of a shielding structure in the form of adjacent triangles, and to use this iris-type collimator design to achieve an approximately circular irradiation field—which comes closer to the shape of the area to be actually irradiated—so that the excess irradiation of approx. thirty percent caused by the corners of the previously square-shaped beam collimation can be avoided. The remaining excessive irradiation area and the formation of penumbrae is not a serious problem, since radiation refers to X-rays for imaging purposes, but not to therapeutic irradiation with beams of much higher energy. As the diaphragm leaves used for X-raying are considerably thinner than those for therapeutic radiation, it is sufficient to provide the proposed iris-type collimator with diaphragm leaves located near the focus which only roughly collimate the beam emitted from the radiation source by preventing afocal rays in the plane of the anode disk of the X-ray source, so as to avoid the emission of radiation from the device to the surrounding environment. By its collimation to a round irradiation field, the beam is sufficiently restricted, since the radiation for imaging purposes should not at all be limited as exactly as a therapeutic beam. After all, the neighboring tissue shall also be displayed so that the doctor can assess the position of the tumor in relation to the surrounding tissue.
For the use of ionizing, i.e. high-energy beams suitable for tumor treatment in this context, publication DE 15 89 432 A1 suggests the use of a collimator with adjacent wedge-shaped radiation shielding units that can be shifted in one plane so as to enable the formation of hexagonal, octagonal or rectangular openings, i.e. a polygon according to the number of diaphragm leaves. However, this type of collimator represents the shape of a tumor quite insufficiently and it does not offer a solution for the formation of penumbrae caused by the front edges of the diaphragm leaves which are not aligned in the direction of the optical path. The prevention of leakage radiation is also insufficient. Although the areas where the diaphragm leaves meet are provided with inset tongues, these tongues are too thin for an effective shielding of radiation emitting through the gap between adjacent leaves. If large irradiation fields are to be treated where the optical path is quite sloping in relation to the borders of the shielding material, the penumbra region will be quite large.
DE 10 37 035 B also refers to a collimator of the type stated in the first-mentioned publication and suggests, for high-energy therapeutic beams, to divide the four beam collimating blocks into two parts along an oblique line, whereby the line shall extend to that point where the inner and end surface (i.e. the surface adjacent to the next block) meet. This will divide each block into a primary and a secondary part which can be shifted towards each other. This allows the formation of different contours and reduces the amount of exceeding radiation compared to square-type radiation collimation. This type of reconstruction of the tumor shape or any other area to be irradiated is certainly quite inadequate and the penumbra problem also remains unsolved. It is soley the problem of leakage radiation that is prevented by the mutual dovetail-type guides in the bordering areas of the diaphragm leaves.
One solution for the penumbra problem is, after all, stated in DE 15 64 765 A1. This publication also sets out from a collimator of the same type as described in the first-mentioned publication with four adjacent radiation restricting blocks which each can be moved in one plane. This system is aimed at achieving a clearly outlined field, i.e. an area without penumbrae. For this purpose it is suggested to design the blocks with swivelable bearings and a mechanism that ensures that the guiding ends that form the border for radiation will be oriented towards the radiation source in each setting. In this way, the radiation will be fully shielded by the material of the blocks. However, this collimator only allows formation of square-shaped irradiation fields, so that large exceeding radiation areas at the edges had to be accepted.
Both the problem of leakage radiation and the penumbra problem are dealt with in FR 2 524 690 A. For the prevention of penumbrae, this publication suggests the use of adjacent plates each of which can be moved in one plane and simultaneously be turned, whereby the different plates are arranged in several planes, so that a graded beam collimation aperture in the shape of a frustum of a pyramid can be achieved. With this structure, the formation of penumbrae can be avoided to a large extent. Leakage radiation is prevented, because the contact points of adjacent plates no longer align, since the individual apertures are in different planes. In closed condition, however, they will still be aligned, so that the radiation source must be switched off or shielded in this state. One disadvantage of this collimator is the intricate mechanical design required for the correct shifting and simultaneous turning of the plates in all planes so as to achieve a beam collimation aperture in the form of the frustum of a pyramid. One further disadvantage of this solution is that the collimation field for irradiation can only be formed on the basis of polygons—depending on the number of plates—but the real tumor contour cannot be accurately defined. Because of the intricate mechanism, a quadrangular beam collimation will be favored. This shape, however, deviates considerably from the real contour of the tumor.
To improve the imaging of the tumor shapes and, in particular, to reduce exceeding irradiation to a minimum when using shielding material of the relevant thickness, one proceeded to the use of exchangeable fixed collimators. With this technique, the tumor shape was captured from different three-dimensional directions, but it required the production of several fixed collimators for each irradiation treatment which could then be used for irradiation from different directions. This included the advantage of accurate contouring and the possibility of adjusting collimation exactly to the optical path so that no penumbra occurs. The disadvantages included the complicated procedure with the continuous exchange of collimators, a prolonged use of expensive devices, as well as the costs for the production of a large number of collimators for each irradiation process; these collimators could not be used any further, since they were designed for one specific patient and could be used only within a very limited time frame even for this patient; the latter resulting from the continuous change of a patient's tumor by growth, regression or deformation.
In order to minimize these costs and efforts, multileaf collimators with a large number of tiny, closely adjacent leaves were created which were suitable to image the tumor by adjusting the leaves accordingly. At first sight, these multileaf collimators had the advantage of a quick setting for any type of shape, however, the mechanical design with the adjusting elements for each leaf was a considerable disadvantage, as well as the more or less large penumbra which occurred at each border of the irradiation field caused by a leaf, depending on its distance from the axis of the optical path.
EP 1 153 397 B1 suggested to prevent the occurrence of penumbrae by providing the leaves with adjustable front edges, whereby a specific mechanism ensures their parallel alignment with the optical path. This, however, requires an even more complicated mechanical structure of the multileaf collimator.
In order to avoid these intricate mechanisms and to increase the flexibility in the shaping of the surface to be irradiated, the publication DE 199 22 656 A1 suggested a scanning system with a collimator aperture which is small enough to irradiate the areas of the object to be irradiated with sufficient precision (FIG. 3). Although the collimator opening of the above proposal is small enough and prevents the formation of penumbrae, it requires extensive time for the scanning process—with a large diaphragm aperture the scanning process can be completed earlier, but the required accuracy cannot be achieved. The use of multiple-hole plates to generate a beam of several scanning rays (FIGS. 5 and 5a) did not lead to satisfactory time reduction. The multiple-hole plate was not flexible with regard to the surface to be irradiated and for an exact irradiation of the marginal areas even smaller apertures had to be used, i.e. the plates had to be exchanged.
In order to increase the scanning speed without reducing the high level of accuracy, the publication DE 101 57 523 C1 suggested the use of a collimator with several collimator apertures of different size which can be brought into the optical path as desired. This was preferably effected with the help of a revolving turret-type mechanism that turned a round plate with different apertures. The high-energy rays as applied in radiotherapy today require a shielding material of 6 to 10 cm in thickness. This leads either to a collimator of considerable weight or the number of aperture sizes must be limited, for example, to three openings. But even with these restrictions, the unused apertures must be covered so as to avoid deficiencies in the shielding of areas through insufficient material thickness. Apart from the plate with the apertures, this system requires an additional shielding plate, also of several centimeters in thickness. For this reason the collimator will be relatively heavy, thus increasing the requirements on guides and drives accordingly. The above stated reasons involve a further disadvantage of this collimator, since only a few defined collimator apertures are available so that the variability of beam collimation is quite limited. In particular, the reduced number of openings makes it impossible to use large apertures of different diameters which could be used to treat one larger surface of the area to be irradiated first and then to irradiate the marginal areas with graded, smaller beams. Since the exposure time for radiation application is several seconds for each point on a surface, the scanning of a surface with fixed beam parameters requires more time than with optimal adjustable values. This applies, in particular, if the beams are narrower than would be possible with regard to the irradiation surface. This leads to an extension of the total treatment period. This is not only inconvenient for the patient who must be immobilized, but it also reduces the number of treatments possible with one device—an aspect of great economic significance in view of the high purchasing and operating costs of these devices. Apart from this, the application accuracy in marginal areas is limited, which is critical if areas with nerves are in the vicinity.
Eventually, EP 0 382 560 A1 suggested an irradiation device of the same type as described at the beginning of this document. This publication refers mainly to a combination of imaging and irradiation treatment. Among other things, it proposes the use of a collimator with an iris diaphragm which offers quite a variable configuration of the aperture. The iris diaphragm, however, produces a polygonal cross-section for the beam. An application from different directions will transform this polygonal cross-section, for example, into a hexagonal shape so that the three-dimensional orientation will continuously change and would have to be included in the calculation of the application with regard to spatial angle, aperture size and irradiation period. This would cause a considerably increased complexity with regard to the calculation of the great number of individual irradiations to be applied—which often sum up to more than one hundred for one single irradiation session—and when considering the above-stated parameters aimed at achieving a specific three-dimensional irradiation profile. But even if the different orientations of the polygonal beam shape were accepted without eliminating them, this would lead to errors in the irradiation profile applied. These errors would increase with the deviation of the polygonal shape from a circular form. Of course, an iris diaphragm could be equipped with more leaves, so that the polygon could be included in calculation as a circle without causing major inaccuracies; this would considerably simplify the calculation process so that the computer capacity and/or computing time could remain within acceptable limits. As a consequence, however, integration in one diaphragm would require enormous mechanical efforts which, from a certain number of diaphragm leaves including drive and guide mechanisms for each leaf, would soon reach the limit of spatial accommodation.
In addition, the solution suggested in EP 0 382 560 A1 shows some further problems:
In order to shield high-energy therapy radiation with a energies in a megavolt range as used today, the shielding material, most often made of tungsten, must have a thickness of between 6 and 10 cm. Since as many leaves as possible shall be used, the thickness of these leaves does not allow to put them in multiple layers one on top of the other. It is therefore necessary to provide them with adjacent side faces as proposed in many of the above-stated publications. Even if these side faces are intricately worked, a small gap will remain and cause an emitting of leakage radiation. This could be prevented by tongues covering the gap as proposed in DE 20 53 089 A, but in view of the great number of required leaves this would increase complexity and require further installation space, since the tongues must be adequately thick so as to ensure adequate shielding properties.
The big advantage in using this type of iris diaphragm, i.e. the formation of beams with different cross-sectional areas—thus offering a significant reduction in the total irradiation time—is at the same time its disadvantage: depending on the aperture size, the beam collimating areas of the iris diaphragm are simultaneously the sliding surfaces between the individual leaves. This means that they must be mainly in perpendicular alignment to the optical path with only minor deviations. When using shields with a leaf thickness of between 6 and 10 cm, a wide-open iris diaphragm has the effect that the rays in the outer beam region will considerably diverge and can be shielded only to some extent, since they are partly outside the material. The rays passing completely through the aperture will be surrounded by a sort of halo, i.e. the penumbra stated further above, that reaches up to the fully shielded area. With an irradiation device of the type described at the beginning of this document, a larger degree of associated irradiation of the surrounding tissue must be accepted—which is, of course, intolerable from a medical point of view—or the marginal areas must be scanned with a very thin beam with only little penumbra formation and which must be graded in such a way that the penumbrae applied by the large beam can be eliminated again as exactly as possible. However, this procedure would multiply the time required for processing and computation.
Apart from the use of an X-ray scanner for image generation, the most significant disadvantage of the irradiation device according to EP 0 382 560 A1 is, however, that it mentions a type of “scanning movement” by the therapy beam which is put into more concrete terms only insofar that an attempt is made to use the iris diaphragm to gain an approximate image of the object's shape by the form of the beam cross-section from each spatial angle. This implies only an overlay of the individual applications from different spatial angles but not a combination of individual applications as performed with the scanning procedure described in DE 199 22 656 A1 and DE 101 57 523 C1. Although this enables the formation of a round irradiation area, as well as the formation of irradiation regions with an elliptical cross-section (as shown in FIG. 4 of the above-mentioned publication), irregular regions, which the object to be treated usually are, cannot be achieved by a combination of individual applications in this manner.
Hence, one basic objective of the invention is to further develop an irradiation device and a collimator of the type described above, so as to generate a three-dimensional irradiation profile that can be applied with greatest possible accuracy and with maximum protection of the surrounding tissue while the mechanical efforts and the time required for computing and irradiation are kept to a minimum.